1. Field of the Invention
The present invention relates to an image forming apparatus and image forming method that feeds and transports a medium while recording an image, or the like, on the medium being transported.
2. Description of the Related Art
Conventionally, some printers, which are one of image forming apparatuses, are provided with an automatic sheet feeder (hereinafter, referred to as ASF (Auto Sheet Feeder)), which are, for example, disclosed in Patent Document 1, Patent Document 2, and the like. A sheet of paper set on the ASF, when print is going to be initiated, is automatically fed to a print start position by driving the ASF, thus setting a leading end position.
The ASF includes a paper feed guide, a hopper, and a paper feed roller. Multiple sheets of paper may be mounted on the paper feed guide. The hopper is tiltable so as to push out the sheets of paper, which are stacked on the paper feed guide. The paper feed roller is arranged in proximity to the lower end of the hopper so as to be opposed to the hopper. The paper feed roller is driven by the same driving source that drives a paper transport roller and a paper discharge roller. As the paper feed roller is driven by the driving source to rotate one revolution while its circular arc surface is in contact with an uppermost sheet of paper among the sheets of paper stacked on the hopper, the uppermost sheet of paper P is fed to the paper transport roller. In addition, paper return levers are provided in a paper feed opening that is formed between the paper feed roller and the hopper. The paper return levers are tilted to a rest position to ensure a paper feed opening for a sheet of paper when paper feeding is initiated, and, when the paper feeding of the uppermost sheet of paper is finished, the paper return levers are tilted up from the rest position and pivotally returned so that the following sheets of paper, the distal ends of which enter a transport path below the paper feed roller, are pushed back onto the hopper Conventionally, there has been a case where a stepping motor is used as the same driving source (paper transport motor) for both the paper transport roller and the paper feed roller.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2003-104578
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2006-89221
Incidentally, the stepping motor is driven in a paper feed mode during paper feeding, while the stepping motor is driven in a driving mode, called paper transport mode, for transporting a sheet of paper while printing after the paper feeding. However, there has been a case where, when a sheet of paper is fed to a leading end setting position, the paper feed roller has not completed a roller reset operation, by which the paper feed roller is returned to an original reset position, and the hopper and the paper return levers are still performing a return operation. In this case, the paper feed roller gradually pivots every time the printing sheet of paper is transported to thereby progress the reset operation; however, at this time, the hopper is also pushed into the rest position against the urging force of a spring and, in addition, an operation to raise the paper return levers is also performed. Therefore, a load generated for the reset operation accompanied by these return operations has also been applied to the stepping motor.
When paper transport is driven in a normal paper transport mode in a state where a large load resulting from the return operation of the hopper or a large load for returning the paper return levers is applied, there has been a case where a step-out of the stepping motor occurs. That is, there has been a problem that a desired amount of paper transport cannot be performed because of the step-out of the stepping motor during paper transport driving or a phenomenon occurs in which a fed sheet of paper is pulled back in a counter paper transport direction due to the step-out during a stop between adjacent paper transport operations and, thereby, deviated from an appropriate position.
Thus, because a load resulting from the above return operation is generated depending on a leading end setting position of a sheet of paper, in order to bypass the above problem, it has been necessary to uniformly set a high torque for paper transport driving in conformity with a maximum load. In this case, a step-out of the stepping motor may be solved; however, because the stepping motor is driven with a high torque even in a paper transport region after the reset operation, where the high torque is originally not required, there have been a problem that the stepping motor heats up more than necessary in order to apply a redundant electric current and/or a problem that drive noises increase and thereby a noise of the printer while printing increases.
The invention is contemplated to solve the above described problems and its object is to provide an image forming apparatus and image forming method that can prevent a decrease in image quality by preventing a step-out of a driving source, which occurs because a load of a feeding device is applied when a medium is transported for image formation.